In recent decades, there has been a growing interest in operating aircraft at maximum efficiency. Efficient operation generally requires flying at or near the minimum drag speed and at high altitude. For these flight conditions, conventional vertical flight path and speed control techniques present significant problems For example, the aircraft's vertical flight path is difficult to stabilize using the conventional technique of maintaining constant thrust and adjusting elevator position. The difficulty in stabilizing vertical flight path stems from the fact that, at minimum drag speed and high altitude, lift increase due to pitch rotation is less long term then lift loss to speed loss. This causes speed divergence and ultimately flight path divergence.
Efforts to solve the problem of maintaining flight path and speed stability have been mainly directed toward the development of independent autothrottle systems to control airspeed. The approach of using an autothrottle system independent of flight path control has proved to be less than satisfactory, particularly with regard to throttle activity. At high altitudes, rapid speed control with an autothrottle requires unacceptably large throttle excursions. The large excursions are required because the thrust increment obtained for a given throttle displacement decreases with altitude and, at high altitudes, is only a small faction of that obtained at low altitude. In unsteady air mass conditions, the problem of large throttle excursions is especially great because of the high frequency of speed deviations caused by the unsteady conditions.
A number of refinements of the approach of an independent autothrottle have been developed in an effort to reduce throttle activity and thereby obtain increased fuel efficiency. A throttle control system in which the turbulence induced components of airspeed error and inertial longitudinal acceleration signals are cancelled to reduce throttle activity in turbulence is disclosed in U.S. Pat. No. 3,840,200, granted Oct. 8, 1974, to A.A. Lambregts, a co-inventor of the present application; and U.S. Pat. No. 3,892,374, which issued to the same inventor on July 1, 1975. A further refinement of autothrottle control by the same inventor is the subject of U.S. Pat. No. 3,989,208, granted Nov. 2, 1976. The invention disclosed in that patent is based on the principle of conservation of the total energy of the aircraft and the inherent transfer of energy from altitude to speed and vice versa. The approach of the invention was to correlate speed errors to altitude errors and to allow the speed errors to be compensated by the altitude control. This in principle reduces throttle control demand but in practical application was not fully satisfactory. Within the context of an independent autothrottle system, the approach had significant limitations. The amplitude of the compensation signal had to be limited, and it was difficult to reference the compensation signal in a system that was independent from the autopilot. In addition, the energy compensation approach cannot make up for the variation in the aircraft total energy due to unsteadiness in the atmosphere.
With the development of the invention disclosed in the last-cited patent, the inventor thereof perceived that the conventional approach of independent autothrottle speed control and autopilot flight path control had reached the point of diminishing returns This realization led to the development of the integrated speed control and flight path control system disclosed in U.S. Pat. No. 4,536,843 granted Aug. 20, 1985, to A.A. Lambregts. In the integrated system, cross-over inputs from flight path to the thrust control and from speed to the elevator control are employed to obtain simultaneous speed and flight path control based on kinetic and potential energy principles. The system generates a total energy rate error signal and an energy rate distribution error signal, each of which has a flight path component and a speed component. The aircraft thrust control is operated to control the total energy state and reduce the total energy rate error to zero. The elevator control is simultaneously operated to control the distribution of energy between potential energy (altitude) and kinetic energy (airspeed) and reduce the energy rate distribution error to zero. This integrated approach has proved to be very successful in avoiding many of the deficiencies and limitations of the conventional approach of independent autothrottle and autopilot control systems and in achieving significant gains in performance. The performance gains include excellent speed and flight path control, elimination of undesired throttle activity, and enhanced fuel efficiency. As disclosed in the patent, the system reduces speed and flight path errors to zero at the same rate (with the same response dynamics) and specifically avoids coupling flight path control and speed control. In other words, adjustments to correct speed errors do not create errors in flight path and vice versa.